1. Field of the Invention
The present invention relates generally to the field of joining dissimilar metals by welding and more specifically to the field of joining dissimilar metals by welding to produce a weld joint which can be glass or enamel coated wherein such coatings will not be subject to the formation of defects caused by the differing rates of temperature differentials which induce expansion and contraction between the dissimilar metals which are joined by such a weld joint.
2. Background
In the chemical processing industry, chemical reaction vessels are used. Such vessels may be formed of mild, or low-to-medium carbon steels of various grades. For example tanks, generally in the form of cylindrical sections, with heads enclosing the ends; are a common arrangement for chemical reaction vessels. The heads are generally welded to a hollow cylindrical section which is formed from rolling steel plates. Ports are then generally formed by welding various conduit attachment devices, such as for example flanges, to apertures formed through the walls of such a chemical reaction vessel. Such apertures can be formed through the heads or through the hollow cylindrical sections, as desired. The purpose of such ports is for providing means of ingress and egress of the materials to be processed within the reaction vessel, for the purpose of insertion of processing equipment into the reaction vessel, for the purpose of inserting measuring apparatus into the reaction vessel and for the purpose of providing openings through which the interior of the reaction vessel can be inspected, maintained and cleaned.
Such chemical reaction vessels, in certain circumstances, are provided with a glass or enamel coating on the interior thereof, normally through known processes and techniques which fuse the glass or enamel to those metal surfaces which form the interior walls of such vessels, as well as to the interiors of the adjacent conduit attachment devices. The purpose of such glass or enamel coatings is to prevent or inhibit the adverse effects produced from the chemical reactions and processing, as carried out within such vessels, from directly attacking the metal used to form those vessels. Such adverse affects can be, for example, corrosion, erosion and/or abrasion.
In many situations, chemical reaction vessels are required to operate under condition of elevated pressure within. For a chemical reaction vessel to properly contain such elevated pressures, in situations where the adverse effects produced from the chemical reactions are present, critical design criteria are required. Firstly, the steel, which forms the hollow cylindrical section of the body of the chemical reaction vessel, must be of special quality with carefully controlled ranges of carbon and alloying elements and free of flaws such as porosity, laminations and cracks. Tight controls must also be applied in annealing and stress relieving such steel. Also, when such steel is welded to itself, special techniques and controls must be implemented to insure high integrity weld joints.
Secondly, due to the special requirements and techniques used to form the heads in dished shapes, different compositions of steels, containing different carbon levels and types and/or quantities of alloying elements, from those used in the steels of the vessel body, may be required. In some circumstances, the heads may be required to be formed of low to medium alloy steels, as distinguished from mild steels.
Thirdly, the conduit attachment devices may be required to be formed of yet other compositions of steels or other metals, including differing levels of carbon content and different qualities and/or compositions of alloying elements.
In fabrication of such reaction vessels, the heads are usually welded to the ends of the hollow cylindrical body section of the reaction vessel, forming a closed tank. The conduit attachment devices are, likewise, welded to the heads and/or the body section. In many cases, such welds join what can be considered dissimilar metals to each other.
Once the reaction vessel has been welded together and all of the interior weld surfaces ground smooth, a glass or enamel coating is applied to all areas which will be exposed to the chemical reactions and processing which the chemical reaction vessel is designed to contain. Of course, such glass or enamel coatings must also cover the weld joints.
The glass or enamel coatings, fused onto the metals, are formulated to be capable of relatively uniform expansion and contraction when exposed to temperature differentiations. Those glass or enamel coatings are also formulated to generally match the coefficient of expansion and contraction of the metals to which they are fused. The problems occur at the weld joints. Although the coatings are capable of withstanding uniform stresses which are imposed in generally linear directions, localized irregular and/or erratic stressing frequently causes the coating to crack. Such stresses are caused by the welding of the dissimilar metals together, due to the irregular and relatively erratic interrelationships of the enlarged dendretic structures which form adjacent to the weld joints, as dissimilar metals are intermixed by the welding operations. The stresses are also caused by the differing coefficients of expansion and contraction of the dissimilar metals, and become more exaggerated as the weld joints become wider and deeper, as will be found in joining thicker metal sections. Because of the irregular dendretic structures and erratic localized grain boundries, the weld joint expansions and contractions are not generally linear, but rather, form complex stress patterns such differ from point to point throughout the weld joint in an erratic, irregular non-repetitive pattern. Such stress patterns cause or enhance the propensity of the glass or enamel coatings to crack and flake off, thus exposing the underlying metal surfaces to the adverse affects of the chemical reactions occurring within the vessel.
There is a need for a design of a weld joint between dissimilar metals, as characterized above, which does not form the irregular and exaggerated dendretic structure along the grain boundries of the weld zone, thus eliminating or significantly decreasing the occurrence of complex localized irregular stress patterns and tending more toward consistent linear stress patterns on the occurrence of temperature differential induced expansion and contraction. Such is provided by the present invention.